Competition in crabs, as in other animals, may be exploitative, where use of a common resource by one organism denies its use to another organism, or interference, where one organism actively prevents or interferes with another organism's use of a common resource. It can occur interspecifically or intraspecifically, and may involve food, mates, space, or shelter. Most attention in the scientific literature has been paid to sympatric, congeneric pairs of species competing for space, and to hermit crabs fighting over shelter in the form of shells.

This section starts with studies of west-coast crabs in general and concludes with a section on invasive green crabs Carcinus maenas

Research study 1

Two ecologically similar grapsoid crabs Pachygrapsus crassipes and Hemigrapsus oregonensis coexist along much of the California coast. Their foraging zones and vertical distributions of burrows overlap, both are omnivorous and eat similar plant and animal matter, and both are nocturnal. In Goleta Slough, a tidal marsh area near Santa Barbara, there is strong competition for burrow space along the banks of the channels. The larger species P. crassipes does not dig its own burrows, but moves into and enlarges ones dug by H. oregonensis.Note in the histogram at the Left that at one location where Pachygrapsus is rare at lower tidal levels, but common at higher levels, numbers of Hemigrapsus are reciprocal. Hemigrapsus falls prey to Pachygrapsus, but at lower tidal levels is provided partial refuge in its burrows. Laboratory tests show that juvenile Hemigrapsus are also able to tolerate conditions of siltation and low salinity that Pachygrapsus cannot (see graph on Right).The author concludes that the species are able to coexist through a balance of predatory/competitive capabilities of Pachygrapsus and burrowing/physical stress-tolerance capabilities of Hemigrapsus. Willason 1981 Mar Biol 64: 125.Photo of Pachygrapsus crassipes courtesy Jackie Soanes, Bodega Marine Laboratory, California

NOTE mean burrow size in the upper region is 10 cm2, 3 times that of the middle region, and 5 times that of the lower region

Research study 2

Five species of spider crabs Loxorhynchus crispatus, Pugettia producta, P. richii, Mimulus foliatus, and Scyra acutifrons inhabit kelp-forest habitats along the central California coast. Their numerical abundances and apparent similarily of habitats suggest the potential for strong competitive interactions within members of the guild. However, a comprehensive study at Hopkins Marine Station, Pacific Grove, California shows that in the major dimensions of body size, microhabitat occupation, and foods eaten, sufficient differences exist to suggest that interspecific competiton is minimal. First, as illustrated in the drawings above, the body size at maturity of the 5 species spans an order of magnitude in carapace width from small S. acutifrons (1cm) to large L. crispatus (10cm).

Second, habitat utilisation varies from narrow, such as P. producta mostly on kelp plants, to broad, such as M. foliatus on many different substrata including an important refuge in kelp holdfasts (see histogram upper Right).

Finally, analyses of stomach contents show partitioning of food resources from dietary generalists such as L. crispatus foraging on a broad range of invertebrates and giant kelps, to strict dietary specialists such as P. producta grazing on kelp Macrocystis pyrifera, to M. foliatus and P. richii capturing drift pieces of the same kelp (see histograms on Left). Scyra acutifrons is an intermediate dietary specialist that feeds on detritus, sponges, and pieces of kelp, and Loxorhynchus crispatus also exploits a wide range of foodstuffs. Niche separation in the spider-crab guild is therefore multidimensional, with overlap in one resource generally being compensated by different utilisation of another resourch. Hines 1982 Ecol Monogr 52: 179.

CLICK HERE to see a video of a kelp crabPugettia producta living in a kelp habitat off the west coast of Vancouver Island, British Columbia.

NOTE the video replays automatically

Research study 3

Two porcelain-crab speciesPetrolisthes eriomerus and P. cinctipes often live sympatrically on west-coast shores. Petrolisthes cinctipes lives in mid- to high-intertidal areas, preferring the protection of tumbled rocks or mussel beds (in which densities may reach 4000 . m-2). Here, the crabs are exposed to air at almost every low tide and spend about half their time emersed. In comparison, Petrolisthes eriomerus occupies low-intertidal and shallow subtidal areas and spends most of its time immersed. Stillman & Somero 1996 J Exper Biol 199: 1845. Photo courtesy Dave Zitten, UBC.

NOTE an easy guide to distinguishing the species is that P. cinctipes has burgundy-coloured antennae, while P. eriomerus has straw-coloured antennae

Research study 4

Although there is some overlap in distributions of the 2 species Petrolisthes cinctipes and P. eriomerus, for the most part they are spatially isolated. Studies in Barkley Sound, British Columbia show that this isolation begins at larval settlement, and thus is not primarily a product of post-settlement competition for space. This is tested by enclosing patio blocks (30cm dia) in mesh cages and placing the cages at high (1-1.5m above chart datum) and low (0.3m) intertidal levels at different sites during mid-summer when megalopae are abundant in the plankton. Half the high-level cages receive 10 Petrolisthes cinctipes (5 males, 5 females) and the other half are crabless (CONTROLS). Similarly, half the low-level cages receive P. eriomerus and half are left crabless. Other low-level cages receive P. cinctipes adults (equal sexes). After 45d the number of megalopae clinging to each patio block is counted. The hypothesis to be tested is that the megalopae settle preferentially with conspecific adults.

Results show that in high-intertidal cages containing P. cinctipes adults there are significantly more P. cinctipes megalopae than in CONTROL cages (see graph above Left). Essentially no P. eriomerus megalopae settle in the high cages. Similarly, in low-intertidal cages, fewer P. eriomerus megalopae settle in cages with P. cinctipes adults or in CONTROL cages, than in cages containing P. eriomerus adults (see histogram on Right).

Finally, significantly more P. cinctipes megalopae settle in low-intertidal cages containing P. cinctipes adults than in CONTROL cages or in cages containing P. eriomerus adults (see graph lower Left). In summary, the megalopae preferentially settle with conspecifics regardless of intertidal height. This settlement strategy gives a “kick-start” to the spatial separation of the 2 species, without the newly settled juveniles being subjected to long, hazardous, and possibly fatal migrations up or down the shore. Near the time of settlement the megalopae tend to be found close to the water surface. Thus, on incoming tides the larvae would wash over rocks containing conspecific adults, first, the lower dwelling P. eriomerus, and then, the higher dwelling P. cinctipes. The larvae presumably perceive species-specific chemical cues emanating from the adults, and drop down to join them. Jensen 1989 J Exper Mar Biol Ecol 131: 223; Jensen & Armstrong 1991 Mar Ecol Progr Ser 73: 47.

NOTE comparable high-intertidal cages with
P. eriomerus could not be tested because
the adults would not have survived

NOTE supportive data for this are provided in later laboratory experiments at the Bamfield Marine Sciences Centre, British Columbia showing that P. cinctipes megalopae swim less in the presence of conspecific adults than in control treatments where adults are absent (see graph on Right). The megalopae are attracted to adults and settle within 2-4d. If no adults are present, then settlement is delayed and the megalopae swim for a further 2-3wk

Research study 5

What regulates the distribution of the 2 species of porcelain crabs? Reciprocal caging experiments at Neah Bay, Washington at high and low levels on the shore show that no P. eriomerus survive at the high level, while P. cinctipes survives at both levels over a 1-mo period.Auxiliary gas-exchanging membranes on the walking legs are lacking in P. eriomerus, and death may be from drying of the gill surfaces in the branchial chamber. Laboratory experiments over 24h show that the low-dwelling P. eriomerus is intolerant of warmer temperatures (20-25oC), temperatures easily tolerated by P. cinctipes (see graph on Left). Other confrontation-type experiments in the laboratory show that P. cinctipes is competitively dominant over P. eriomerus. These features appear to set the upper limits of P. eriomerus, but what sets the lower limits of P. cinctipes?

A clue to answering this question is the observation by the authors that P. cinctipes tends to inhabit beds of mussels Mytilus californianus, an area relatively free of water movement and sediments, and this is where greatest densities are reached (4000 . m-2). The following experiment in Barkley Sound, British Columbia suggests specifically that the lower distribution of P. cinctipes may be limited by avoidance of sandy substrata. Several vertical arrays (4.3 x 0.6m) of boulders are placed on a beach, with the mid-point of each array corresponding with the normal line of vertical separation of the 2 species on natural rocky beaches. The upper boulders are resting on pebbles; the lower ones, on sand. In the lower portions of half of the arrays (the zone normally occupied by P. eriomerus), concrete blocks are inserted to raise the boulders off the sand. These are the EXPERIMENTAL arrays. The same number of arrays is left unmodified as CONTROLS. One hundred adult P. cinctipes bearing numbered tags are then released into the top portions of each array and left undisturbed for 1mo. After 1mo the numbers of crabs are counted. in the EXPERIMENTAL arrays the results show that P. cinctipes have moved downwards into the “eriomerus” zone, while in the control arrays they are mostly limited to the upper parts where the boulders are not resting on sand. The authors conclude that avoidance of sand may be a factor in setting the lower limit of distribution of P. cinctipes. Jensen & Armstrong 1991 Mar Ecol Progr Ser 73: 47.

Research study 6

Casual observation of a mixed kelp forest would suggest that different species of kelp-inhabiting crabs, such as Pugettia gracilis and Oregonia gracilis, are distributed more or less randomly, but this is not the case. Monthly SCUBA surveys at 6 habitats in Kachemak Bay, Alaska over a 16mo period by researchers from the University of Alaska Fairbanks show that each species has its own special microhabitat-resource preferences, and that these tend to keep them spatially separated. Both species decorate by attaching bits of algae on their carapaces for protective camouflaging, but they differ in the extent to which they do this. For example, Pugettia gracilis decorates relatively little and seems more dependent upon the structural complexity of the kelp to provide protective cover. Thus, its location and abundance vary with seasonal changes in kelp structure. In comparison, Oregonia gracilis has complex and extensive camouflaging decoration and is less reliant on being within the 3-dimensional canopy for cover. This last species is most abundant in understory habitats, especially in late summer. Daly & Konar 2010 Crustaceana 83 (6): 659.

NOTE several other decapods are present at the sites, but the 2 species listed are by far the most common

A kind of “made-up” laboratory-based study on Hemigrapsus spp. at the University of Delaware deals with potential competition between 3 species of Hemigrapsus (the indigenous west-coast H. oregonensis and H. nudus, and the non-native east-Asian H. sanguineus). However, as the last species appears not to be established on the west coast, the authors premise their study with a “why not” question. Results of pairwise tests for space competition amongst juveniles and adults of these species show that juveniles of H. sanguineus are out-competed for shelter by young of both indigenous species. The authors suggest that this limitation may be preventing colonisation of the species despite abundant potential via ship transport between west coast and Asian ports. Steinberg & Epifanio 2011 J Exp Mar Biol Ecol 404: 57.

NOTE the authors acknowledge that their experimental approach is rather simplistic and that there are many other factors, both biological and physical, that may be operating to prevent colonisation

Invasive green crabs Carcinus maenas

Research study 1

First records of the North Atlantic green crab Carcinus maenas on the west coast date from 1989-90 in San Francisco Bay, California. Its mode of introduction is uncertain, but could involve ballast water, fouling growth on ships’ hulls, imported live bait (and/or imported clawed lobsters), or perhaps intentional release. Distribution by larvae is considered by the authors but dismissed on the basis that the known planktonic larval life of Carcinus is too short (17d at 25oC or 80d at 12oC ) to enable drift from established populations in the North Atlantic or South Africa. Once a “minimum viable beachhead population” size is established, the enormous fecundity of the species, up to 200,000 eggs per year per female, would aid in further recruitment. Carcinus has a catholic diet, with numerous species of algae, seed-bearing plants, invertebrates, and fishes being consumed. Favoured foods are polychaetes, gastropods, and bivalves. In San Francisco Bay, ironically, most of Carcinus’ diet is made up of introduced species, a few of which the crab has encountered in the Atlantic and some of which are new. In view of the species’ aggressive and competitive behaviour, and its preference for bivalves as food, the authors warn of the liklihood of large-scale ecosystem changes in the Bay in years to come. Cohen et al. Mar Biol 1995 122: 225.Photo courtesy Fred Uglow, University of Hull, England.

NOTE green crabs invaded eastern NA in early 1800s and South Africa in the 1980s

Research study 2

The green crab Carcinus maenas is a notably aggressive species. Fortunately, its distribution here on the west coast is limited to more protected habitats. Potential prey populations in more wave-exposed habitats are unlikely to be affected. In subtidal sand/mud habitats, one would predict that green crabs would be out-competed by the larger Dungeness crab C. magister; however, casual laboratory observations indicate that C. maenas will actually prey upon Cancer magister and Hemigrapsus oregonensis of equal or smaller size, and larger-sized Cancer magister appear not to eat the green crabs. However, the 2 species may compete for common prey items. After enclosing single individuals of Carcinus maenas in wire-net enclosures on soft-bottom habitats in Bodega Harbor, California for 12d, then assessing potential prey abundances in experimental and control enclosures, researchers record significant decreases in abundance of bivalves, cumaceans, amphipods (1 species only studied), but not of tanaidaceans, amphipods (1 species), polychaetes, and phoronids, although some trends are apparent. The bivalves, representing 2 species of Transennella, are numerically the second and third most abundantinvertebrates in the cores (to 5cm depth) and represent an important food item of Dungeness crabs. With the exception of the polychaete Exogene sp., most of the other non-eaten species are in low abundances. Grosholz & Ruiz 1995 Mar Biol 122: 239.

NOTETransennella spp. are noted by the authors to reach >10,000 individuals.m-2in various coastal embayments

NOTE tanaidaceans are the most abundant invertebrates, numbering 300/10cm-dia x 5cm-deep core in the control enclosures

Transiennella sp.

Research study 3

So, what is the predicted impact of invasion of European green crabs Carcinus maenas onto west-coast shores and other areas of the world? This is estimated by comparing invasion characteristics for 3 areas now hosting green crabs (western NA, eastern NA, and South Africa) with the same characteristics in its native range in Europe. The characteristics are: 1) habitat usage, 2) diet preferences, 3) size of individuals, 4) rate of range expansion, and 5) demonstrated and potential impacts.

In comparison, diet preferences
show similarity in all invaded areas,
with molluscs, crustaceans, and
annelids being preferred in that
order (see table above Right).

Sizes tend to differ among the invasive populations and, overall, are larger than in the parent populations in Europe (see table lower Left).

Finally, potential ecological impact seems quite varied and probably indicates the strong adaptive potential of green crabs (how else could they make these many invasions?; see table lower Right).

The approach is a nice one but falls short of the author’s expectations mainly through lack of good comparative data. It would be interesting to see how the addition of another decade’s worth of data will add to the picture. Overall, this is a lovely study. Grosholz & Ruiz 1996 Biol Conserv 78: 59.

NOTE inclusion of echinodermata is a bit unusual, in that presumably many other taxa are not present in the diets of green crabs and yet are not shown here

NOTE the authors actually conclude that ecological impact is “relatively similar” across the 3 invasions. However, if “impacts on fishes and birds” is removed as being too uncertain, and “impacts on bivalve molluscs” and “impacts on echinoderms” are removed for reason that they are already included in “diet preferences”, then the remaining entry “impacts on other crabs” is quite varied (ranging from “-“ or no impact to “++” or high impact

Research study 4

In 1993 Carcinus maenas was recorded in Bodega Bay, about 100km north of San Francisco, and in 1997 in Coos Bay, Oregon, several hundred kilometers further north. Carcinus maenas has aggressive feeding habits and has already become a serious pest for mariculturists – particularly those attempting to rear Manila clams Venerupis phillippinarium, themselves introduced into the San Francisco Bay area in the 1940’s. Grosholz et al. 2001 J Shellf Res 20: 913; for another review of potential impact on west-coast fauna and concerns of fisheries scientists see Jamieson et al. 1998 J Nat Hist 32: 1587.

NOTE the species was also observed at Nootka Sound, British Columbia in 2002

Research study 5

A 9-yr study on the effects of green crabs Carcinus maenas on food-web dynamics in Bodega Bay Harbor, California shows that the predator exerts a strong “top-down” control on invertebrates.Most notably, densities of clams Nutricola spp. and shore crabs Hemigrapsus oregonensis decline by up to 10-fold within 3y of Carcinus’ arrival (see graph above Right). The graph shows density of green crabs from their arrival in 1993along with comparative densities of Hemigrapsus and Nutricola during the same period.

Field experiments in which juvenile green crabs are caged with similar-sized H. oregonensis in 0.3m2traps or cloth cages for 1-2d periods show that the indigenous species is no match for the invader.

Gut analyses of actively foraging field Carcinus reveal a preference for bivalves, mostly Nutricola spp. (59% of diet), various crustaceans including crabs and amphipods (40%), algae (10%), and polychaetes (3%). After the arrival of Carcinus, several other sandy mudflat-inhabiting species of polychaetes and tanaid crustaceans increase in abundance, likely as a response to removal of competing species by Carcinus. Although the authors expected some “bottom-up” effects, for example, on shorebirds that rely on benthic invertebrates for food, this was not evident. However, they anticipate that it will occur as the crabs spread further and increase in abundance. The authors summarise their data in a path diagram showing food-web relationships of C. maenas (not included here). Grosholz et al. 2000 Ecology 81: 1206.

Research study 6

In its native habitat, juveniles of the European green crab Carcinus maenas usually live under rocks and shells in the intertidal zone, a location that on the west coast would place them in potential competition with several species of grapsoid1 crabs. However, field sampling in Bodega Bay Harbor, California show that juvenile Carcinus appear to be largely excluded from rock/sand habitat dominated by Hemigrapsus oregonensis (see histogram on Left).

Similarly, in field tests in Bodega Bay Harbor where experimentally placed oyster-shell plots are allowed to be colonised by H. oregonensis2 and C. maenas, the former species also dominates (see graph on Right).In the laboratory, as well, Hemigrapsus consistently dominates equal-sized Carcinus in contests for shelter. However, in arena-type competitions for food, using a screw- and wire-anchored damaged bivalve3 as prey, C. maenas dominates over H. oregonensis. This suggests that the competitive interactions between the 2 species may actually favour H. oregonensis for interference-type space competition, and C. maenas for exploitative-type food competition. Jensen et al. 2002 Mar Ecol Progr Ser 225: 251.

NOTE2 density of H. oregonensis in these experimental plots reach in excess of 600 individuals .m-2

NOTE3 either Mytilus spp. or Venerupis philippinarum

Research study 7

The competitive and predatory impacts of green crabs Carcinus maenas on young Dungeness crabs Cancer magister are investigated at the Bodega Marine Laboratory, California using specimens of Cancer obtained from Coos Bay, Oregon. First, laboratory observations show that green crabs will displace Dungeness crabs of equal size (15mm carapace width) from shelters in one-on-one competition (see histogram on Left). Note that as the night wears on, the green crabs become more succesful at displacing the Dungeness crabs from the shelters.

During night-foraging trials using fresh, damaged clams anchored to the bottom, green crabs also dominate in 2 ways. First, Carcinus is significantly more successful at making contact with the food when Cancer is feeding, than vice versa (see histogram above Right).Thus, Carcinus is able to approach Cancer with about 85% success, versus about 35% for the reverse. Second, once an approach is made, Carcinus is about 2.5 times more successful at displacing Cancer than Cancer is at displacing Carcinus (although note the few actual displacements out of hundreds of trials).

Finally, lab and field enclosure experiments show that Dungeness crabs tend to move out of oyster-shell refuges also inhabited by green crabs and to go into open-sand habitat (see schematic lower Left). Conversely, in both situations C. magister is more likely to remain in the shell refuge in the absence of C. maenas. The authors conclude that invasive green crabs could have an overall negative impact on the indigenous Dungeness crab fishery through predation on young stages and exclusion from protective oyster-shell habitats. McDonald et al. 2001 J Exper Mar Biol Ecol 258: 39.

NOTE the shelters are single clam shells, competed for by 2 crabs, one of each species

NOTE a recent assessment of economic impact of C. maenas on commercial shell-fisheries on the U.S. west coast using a combination of ecological and economic models concludes that past and present impacts are minor. Future impacts, such as into Alaska, will depend upon degrees of density increases and range expansion. Grosholz et al. 2011 Ecol Appl 21: 915; for a slightly different risk-assessment model focusing on conditions at Cherry Point, Washington (not yet invaded) see Colnar & Landis 2007 Human & Ecological Risk Assessment 13: 120.

After its establishment in San Francisco Bay in 1989, green crabs Carcinus maenas have moved steadily northwards. In 1998, coincidental with an unusually strong El Niño-generated current flowing northwards from September 1997 to April 1998, a strong settlement occurred along the coast as far north as Vancouver Island, British Columbia. Studies on this cohort by a consortium of researchers shows, interestingly, that growth is significantly faster than in east-coast and European counterparts (see graph). Note in the graph that west-coast specimens reach 46mm carapace width by the end of the first summer, and 66mm by the end of the second. Comparable sizes for European specimens are 18 and 43mm, respectively. Warmer water temperatures on the west coast certainly lead to more frequent moulting but, as discussed by the authors, there are likely to be other contributing factors, most notably food limitation and higher incidence of parasitism in the more crowded European populations. Green crabs live for only 4-6yr, so the original colonising year class of 1998 has probably completely died out. Northward-flowing currents of the one just described occur only sporadically, but local reproduction and recruitment appear high enough to maintain permanent populations in Oregon and Washington estuaries. Behrens Yamada et al. 2005 Biological Invasions 7: 309; see also Behrens-Yamada & Gillespie 2008 ICES J Mar Sci 65: 725 for further discussion of future prospects of green crabs on the west coast. Photograph courtesy Museum Victoria Australia.

Research study 10

A novel idea with respect to habitat use by green crabs Carcinus maenas in Willapa Bay, Washington is to model their bioenergetics budgets in 4 habitat scenarios to determine which suits their metabolic expenditures most favourably. The researchers input a variety of ecological and physiological parameters published for C. maenas and, in the single case of excretion, for related brachyuran into the 4 models to determine energy expenditures in the different habitats over a 214d simulation period. Results show, interestingly, that predicted metabolic costs are about 6% higher in intertidal habitats than in subtidal ones. The intertidal-simulation crabs, moreover, are less efficient than subtidal ones in converting consumed food energy into growth. So, where do the real green crabs live in Willapa Bay? Monthly trapping reveals that they live predominantly in mid-intertidal habitats and not at all in subtidal ones. The authors credit the (fairly major) discrepancy from model prediction to their failure to include interspecific interactions, such as agonistic ones that may occur with subtidal competitors such as Dungeness crabs. Thus, although subtidal life is predicted to suit C. maenas better, the species may be excluded from it by unfavourable interactions with C. magister. Although laboratory tests of competitiveness of the 2 species (see Research Study 7 above) actually show that Dungeness crabs are inferior to green crabs, such interactions regardless of the outcome would increase metabolic costs for both participants and, for energetic considerations alone, are best avoided. For readers that may have doubts about the overall accuracy and usefulness of such models, the considerable care and attention to detail evidenced in the present study should put some of their concerns to rest. Nevertheless, and as reminded us by the authors, the ultimate degree of accuracy of the models will depend largely on the accuracy of the inputed data for the various parameters. McDonald et al. 2006 Estuaries & Coasts 29 (6B): 1132.Photograph courtesy unknown website.

NOTE these are the high intertidal, mid-intertidal, and subtidal areas, with the fourth being the subtidal area with migrations into the intertidal region

NOTE these include estimates for energy equivalents of consumption, defecation, excretion, and oxygen-uptake rates; air and water temperatures; energy loss to specific-dynamic action; daily tidal cycles; activity “multipliers” during submergence during day and night high tides; energy contents of various bivalve prey; and so on

Green crab Carcinus maenas on
a shallow intertidal substratum 1X

Research study 11

To what extent might the invasional success of green crabs Carcinus maenas depend upon the morphology and functioning of their claws? This question is addressed by a research group in Oregon by comparing claw morphology, mechanical advantages, and feeding rates of green crabs with the same rates in the competitively inferior Dungeness crab Cancer magister. Results show for comparably sized individuals that claw sizes and mechanical advantages are greater for C. maenas than for C. magister (see illustration above Left). Additionally, propal heights are greater in the crusher claw of C. maenas than in either of the monomorphic claws of C. magister. This greater height allows larger-sized prey to be attacked. Closer-muscle masses are also significantly greater in the crusher claw of C. maenas (3.9g) than in either claw of C. magister (1.5-1.7g). Feeding rates, though, are faster for Dungeness crabs when presented with softer-shelled prey such as mussels Mytilus trossulus (see histogram upper Right), but not when offered harder-shelled prey such as native oysters Ostrea lurida (a favoured prey of C. maenas; see histogram lower Right). The authors conclude, then, that feeding rates and overall potential predatory impacts of green crabs will depend upon prey type. Behrens Yamada et al. 2010 J Shellf Res 29: 471.Drawings courtesy the authors and Timothy Sullivan.

How suitable is the newly invaded west-coast habitat for growth of green crabs Carcinus maenas? The answer, based upon a comparison of crab sizes between a “home” location in Anglesey, Wales (53oN latitude) and an invaded habitat in British Columbia, Canada (49oN) is, quite suitable indeed. The British Columbia population (both sexes) is significantly larger than the Anglesey population (by about 40%), suggesting a faster growth rate and thus a potentially greater fecundity. Interestingly, while the home population is comprised of green- and red-coloured morphs (about 45 and 55%, respectively, for both sexes), the invasion population is predominantly green in colour (84% of males and 47% of females). The reddish hues are visible primarily on the ventral surface and legs, and are apparently indicative of a more prolonged intermoult period, or possibly cessation of moulting entirely. Conversely, green coloration is associated with active growth and moulting. The authors discuss possible explanations for faster growth in the invasive population. McGaw et al. 2011 Mar Ecol Progr Ser 430: 235.Photogaphs courtesy Iain McGaw, Ocean Sciences Centre, Memorial University, Newfoundland.

NOTE the area selected for study in British Columbia is Barkley Sound, first colonised in 1999 by just a few individuals, but then more intensively settled in 2005. Water temperatures in this area are similar to those in Anglesey

Most studies on invasive effects of green crabs are short-term, lasting days, weeks, or just a a year or two, and evaluate just a single population-level effect of the invader. Thus, a recent study by researchers at Bodega Marine Laboratory is welcome, for it provides data spanning a 14yr period (1993-2006) on several different effects of invading green crabs Carcinus maenas on shore crabs Hemigrapsus oregonensis. After initial exposure to Carcinus in Bodega Bay, numbers of Hemigrapsus decline (see graph on Left), body sizes decrease, and the population is forced higher up the intertidal zone. Later, when numbers of the invading species decline or otherwise fluctuate, the abundance of native shore crabs rebounds to pre-invasion levels, but this may not be matched by a corresponding rebound to pre-invasion body size, or by a return to previous tidal levels occupied. Note in the graph on the Right that body sizes of Hemigrapsus begin to increase only during the last 2yr of the study, a full decade after Carcinus numbers began to decline. The major finding in the study is that competitive effects on native H. oregonensis are decoupled from short-term changes in numbers of C. maenas, and that these effects may persist for many years – up to a decade as shown in the present study. De Rivera et al. 2011 Mar Ecol Progr Ser 429: 145.

NOTE see Research Study 2 above for a report on an early part of the study done by members of the same research team

Research study 14

A research group from University of Washington investigates competitive interactions of invasive Carcinus maenas with native Cancer productus and C. antennarius in 3 embayments1 in central California (see map). Trap and snorkel surveys show that Carcinus is most abundant in each embayment in shallow areas lacking large Cancer spp. In habitats near to populations of the native crabs, especially those with a narrow intertidal area, Carcinus exhibits high levels of limb damage and loss. Green crabs tethered2 in areas frequented by Cancer spp. experience up to 70% limb loss in comparison with 0% loss in marsh areas lacking the native crabs. The study shows a far greater negative impact3 on incursion by green crabs than found in other investigations. Jensen et al. 2007 Mar Biol 151: 2231.

NOTE1 these are Bodega Bay Harbor, Tomales Bay, and Bolinas Lagoon

NOTE2 the authors discuss the limitations of tethering in such experiments